The cell from A1 to A3 is addressed as A1:A3. Assume A4 is =SUM(A1:A3). When the same formulae is copied to column B4 relative reference automatically change the formulae as B4 is =SUM(B1:B3). Relative here means that the reference to this area will be changed automatically when you copy the formulae.
Absolute references are the opposite of relative addressing. A dollar sign is placed before each column name and row number in an absolute reference, for example, assume A5 is =SUM($A$1:$A$3) when the same formula is copied to B5 reference will not automatically change and formulae for B5=SUM($A$1:$A$3) same as A5.